25-10-2018, 07:37 PM
Craig
Firstly, welcome to the forum. Your post is fine and people on here are great.
I will throw your maths even further into a tizzy. I found a User Manual and right at the end in the specs is...
5. AM IF : 1st IF : 55.845 MHz
2nd IF : 10.700 MHz
3rd IF : 45 KHz
FM IF : 128 KHz
The technique of mixing up is quite conventional as it moves the first IF out of the tuning range hence less image issues. Eg for a receive frequency of 15.000 MHz, the LO will likely be at 15 + 55.845 = 70.845MHz, image at 70.845 + 55.845 = 126.770 MHz but simple filtering solves that one. The thing about these high IF values is that there is only minor selectivity applied there. More IF filtering occurs at 10.7 but the majority is like to happen at 45KHz via DSP in your unit.
There are possibilities other than signal intermod.
Straight front end overload is one - and one of the easiest ways to see if it is this is to attenuate the signals from the antenna where whey enter the radio. A variable potentiometer (/ pot) as a variable voltage divider is enough to find out. Typically FE overload will stop sharply as the signal is attenuated (ie cut out as the pot shaft is rotated). Other desired signals will be there but weaker but the AM BC stations will disappear.
There is a possibility of adding a High Pass Filter at 1.5-1.6MHz to attenuate BC signals but that isn't going to help if you are a BC band DXer. It is ok for HF - say 3MHz upwards.
Similarly, high Q notch filters in shunt across the antenna feed terminals can be used to attenuate the specific BC station frequencies (one or both with a bit of finicky tuning). These are simply an LC series circuit that you tune for minimum signal at the BC stations frequency.
You don't mention the how far physically you are away from the TXs at 720 KHz and 810 KHz and that becomes a critical factor in how you might solve it.
Try the pot idea and advise if the signal problem slowly attenuates or simply cuts out.
73 Doug
Firstly, welcome to the forum. Your post is fine and people on here are great.
I will throw your maths even further into a tizzy. I found a User Manual and right at the end in the specs is...
5. AM IF : 1st IF : 55.845 MHz
2nd IF : 10.700 MHz
3rd IF : 45 KHz
FM IF : 128 KHz
The technique of mixing up is quite conventional as it moves the first IF out of the tuning range hence less image issues. Eg for a receive frequency of 15.000 MHz, the LO will likely be at 15 + 55.845 = 70.845MHz, image at 70.845 + 55.845 = 126.770 MHz but simple filtering solves that one. The thing about these high IF values is that there is only minor selectivity applied there. More IF filtering occurs at 10.7 but the majority is like to happen at 45KHz via DSP in your unit.
There are possibilities other than signal intermod.
Straight front end overload is one - and one of the easiest ways to see if it is this is to attenuate the signals from the antenna where whey enter the radio. A variable potentiometer (/ pot) as a variable voltage divider is enough to find out. Typically FE overload will stop sharply as the signal is attenuated (ie cut out as the pot shaft is rotated). Other desired signals will be there but weaker but the AM BC stations will disappear.
There is a possibility of adding a High Pass Filter at 1.5-1.6MHz to attenuate BC signals but that isn't going to help if you are a BC band DXer. It is ok for HF - say 3MHz upwards.
Similarly, high Q notch filters in shunt across the antenna feed terminals can be used to attenuate the specific BC station frequencies (one or both with a bit of finicky tuning). These are simply an LC series circuit that you tune for minimum signal at the BC stations frequency.
You don't mention the how far physically you are away from the TXs at 720 KHz and 810 KHz and that becomes a critical factor in how you might solve it.
Try the pot idea and advise if the signal problem slowly attenuates or simply cuts out.
73 Doug
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.